There are estimated to be over 120 million land mines distributed throughout the world in 65 countries. Each year, thousands of noncombatants, a large proportion of which are children, are injured or killed by unexploded land mines. Although clearing efforts are under way, the International Red Cross, estimates that, at the present rate of clearance, it will take over 1,100 years and cost over $33 billion (in today's dollars) to clear the currently deployed mines.
The current mine detection technology is partly responsible for the slow mine clearance rate. Currently, deminers and dismounted countermine engineers use metal detectors. Although this somewhat primitive technique is effective for metal housed land mines, many modern land mines are not metal housed and contain a low metal content, which can severely limit the effectiveness of metal detectors. Such mines must be located by “probing” with probes.
“Probing” can be an extremely dangerous and time-consuming method of detecting land mines. This method involves “feeling” for mines by methodically inserting rigid rods into the soil. When a solid object is encountered in the soil, it is then excavated to determine if the object is a mine.
Another mine detection technique is infrared or IR mine detection. Such systems rely on the fact that mines have a different thermal mass than their surroundings and therefore heat up and cool down at different rates. Furthermore, the disturbance in the soil from the burial changes the porosity of the soils, resulting in anomalous water content and a different heating and cooling response to diurnal changes. IR systems are most promising as a support for other technologies, rather than as a stand alone technique. IR sensors are readily available as imaging systems and, when combined with neural networks, can detect individual mines as hot or cold spots on the surface of the ground.
Ground penetrating radar (GPR) is a method of directly imaging buried objects. GPR uses a wideband antenna to irradiate the soil with an electromagnetic field covering a large frequency range. Reflections from the soil caused by dielectric variations are measured and converted into an image. Although promising, this technology has limitations, in particular the resolution required to image small objects requires GHz frequencies which decrease soil penetration and increase image clutter. The high cost of GPR systems also inhibits widespread applications in mine clearing operations.
Other mine detection techniques seek to locate mines by detecting trace levels of explosives in the soil around the buried mines. Because mines are typically of cheap construction and not hermetically sealed, mines buried for long periods can “leak” explosives and derivates thereof into the soil surrounding the buried mine. In time, the concentrations in the surrounding soil can increase to 2-8 ppb w/w for TriNitro Toluene or TNT, the explosive used in over 85% of all land mines. TNT contaminants, such as 2,4-dinitrotoluene and TNT derivatives, such as 2-amino-4,6-dintrotoluene or 2-ADNT and 4-amino-2,6-dintrotoluene or 4-ADNT, may be present at concentrations an order of magnitude greater or more than the primary explosive. Evapo-transpiration in soils can move the leaked explosives and explosive related chemicals to the surface, where the surface microlayer of the soils potentially adsorbs and concentrates the vapors, particularly when the surface dries. The surface microlayer refers to the top millimeter or two of the soil at the soil/atmosphere interface.
Canines are an example of a detection method based on the presence of trace levels of explosives in the surface microlayer around the mine and have been used for land mine detection for decades. Experiments have shown that canines smell the explosives that leak from most types of land mines. Even though canines can be effective for this purpose, their use is not without problems. Logistical problems are significant, and dogs are expensive to train and maintain. Dogs do not perform well under all field conditions. In addition, the performance of the dog can be limited by the skill of the dog handler, the dog's desire to work, and the health of the dog.
Currently, various systems are under development to detect land mines by “sniffing” above the soil for the ultra-trace explosive vapors. This technique can be quite challenging as the equilibrium concentration of the ultra-trace explosive vapors in the soil/atmosphere boundary layer is typically quite low and therefore only minute quantities (10−10-10−12 g) are available for detection. The amount of material that is available to passive vapor sensing systems is limited to no more than the vapor in equilibrium with the explosive related chemicals (ERCs) distributed in the surface soils over and near the land mine. Examples of ERC's include explosive co-contaminants such as isomers of dinitrotoluene and degradation products of the explosives, such as 2-amino 4,6 dinitrotoluene and 4-amino 2,6-dinitrotoluene. Unfortunately, TNT is a solid with a very low vapor pressure under ambient conditions. The TNT, and many other explosives, are crystalline solids and have high boiling points. TNT, for instance, boils with decomposition at about 255° C. Worse, explosives and explosive derivatives (explosive related chemicals or ERCs) aggressively bind to surfaces and soils. The distribution of explosives and ERCs in the soil can be quite heterogeneous. The low equilibrium vapor pressure of TNT in the soil/atmosphere boundary layer and the limited volume of the boundary layer air imply that passive chemical vapor sensing systems require sensitivities in the picogram range or even lower. This can be difficult to implement as the passive chemical sensing system must separate the signal from a highly variable background.
In addition to land mines, explosives and other controlled substances, such as drugs, have become major societal problems. Increasingly, terrorist acts using explosives are becoming a problem not only for countries in the Middle East but also for Western countries in other parts of the world. Drug abuse has been a longstanding problem for Western countries and consumes large amount of law enforcement resources each year. As in the case of land mines, canines, metal detectors, and “sniffer” detectors have been used at various locations, such as airports, border crossings, and the like to detect explosive devices and illegal drugs. These measures have had mixed success for many of the same reasons as their limited successes in detecting land mines.
Another measure that has been employed to detect contraband substances has been to collect loose particles from surfaces or skin with a vacuum cleaner or a swipe. The swipe or the particles collected by the vacuum are then heated to release the vaporizable material for analysis. This approach is in routine use at airports throughout the world for screening airline passengers. An example of such a system is the Barringer™ Ion Scan System™. This technique has drawbacks. The use of swipes or particle vacuums is an intermittent process, which requires manual intervention between the sampling and analysis. This is a time consuming approach that is inherently slow.